1. Field of the Invention
The present invention relates generally to electronic packaging technology and, more particularly, to a three-dimensional semiconductor stack package and a memory module using the stack packages.
2. Description of the Related Art
In general, a semiconductor package is defined as the housing and interconnection of integrated circuits, also referred to as chips or die, to form an electronic system. One of the functions that the package provides is to remove heat generated by the chips or system. Therefore heat dissipation becomes a much more important factor that must be considered during the design or fabrication of the packages, especially in view of recent advances in packages having high integration, high memory density and high speed. A three-dimensional stack package in which several individual packages are arranged is a good example.
FIGS. 1 to 3 show, in cross-sectional views, three types of conventional stack packages. Referring to FIGS. 1 to 3, each stack package 10, 20 and 30 has a two-layered structure (FIGS. 1 and 2) or a four-layered structure (FIG. 3).
The stack packages 10 and 20, shown in FIGS. 1 and 2, includes two individual packages 11a and 11b or 21a and 21b that are arranged in layers. Each individual package 11a and 11b or 21a and 21b has an integrated circuit (IC) chip 13 attached to a top face (in FIG. 1) or a bottom face (in FIG. 2) of a circuit substrate 14. The IC chip 13 is electrically coupled to the circuit substrate 14 via metal wires 15 that pass through a central opening of the circuit substrate 14 and are protected by an encapsulant 16. Mechanical and electrical connections between the individual packages 11a and 11b or 21a and 21b are made by solder balls 17 formed at peripheral parts of the bottom face of the circuit substrate 14.
The stack package 30 shown in FIG. 3 includes four individual packages 31a, 31b, 31c and 31d that are arranged in layers. Each individual package 31a, 31b, 31c and 31d has the same structure as discussed above. The stack package 30 is mounted on a module board 32 via the solder balls 17 of the lowermost individual package 31a. It is known in this art that such a conventional stack package may be configured up to an eight-layered structure.
FIG. 4 shows, in a plan view, a conventional memory module 49. As shown in FIG. 4, several stack packages 40 are mounted on one or both surfaces of the module board 42, thus constituting a memory module having high memory density.
Generally, heat generated from the IC chip in actual use of the memory module 49 escapes toward the module board 42 and toward the air. However, the above-described stack packages have poor heat dissipation because unfavorable air gaps 18 (shown in FIGS. 1 to 3) exist between the individual packages and between the lowermost package and the module board.
FIG. 5 is a temperature distribution diagram produced by simulation of heat dissipation in a conventional stack package. The simulation was applied to two memory modules inserted in a server system, and each memory module used has eighteen stack packages mounted on both surfaces of the module board 42. FIG. 5 shows a pair of upper and lower stack packages 40a and 40b on both surfaces of one memory module.
As appreciated from FIG. 5, the heat dissipation property of the stack packages 40a and 40b degrades around the air gaps. This is caused by relatively poor heat conductivity of air and a heat-trapping phenomenon occurring in the air gap. Such low heat dissipation becomes serious as the number of the individual packages increases. Furthermore, since cooling efficiency is reduced as the width of the air gap becomes narrower; the heat dissipation of the package lowers. As a result, a conventionally used stack package and memory module cannot avoid degradation of reliability due to a rise in temperature.